Method and device for rapid non-destructive quality control of powdered materials

ABSTRACT

A method of non-destructive testing for quality control of powdered materials having dielectric properties based on the use of electromagnetic capacitance techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

Claims priority of Provisional Patent Application No. 61/124,984, FiledApr. 21, 2008

FEDERALLY SPONSORED RESEARCH

None

SEQUENCE LISTING

None

FIELD OF THE INVENTION

The present invention relates to non-destructive testing fordetermination of electrical and associated properties such ascomposition, moisture content, and particle size, of bulk materials, andespecially as related to the quality control of powdered materials suchas cement.

BACKGROUND OF THE INVENTION

Physical and chemical properties of powdered materials may be criticalparameters in a variety of industrial applications. Among many examplesare the construction industry, wherein properties of cement have to becontrolled, or the manufacture of lithium-based batteries whereproperties of the initial powdered electrode active materials must betested, or in case of super capacitors and in other areas. Generalchemical analysis, if conducted for testing purposes, can consume agreat deal of time and is often not practical for rapid quality controlof powdered materials.

The main purpose of the present invention is to provide fastcomprehensive non-destructive 100% quality control of powered materialsof low conductivity, such as cement or cement-based composites, whetherin a containers or on a moving conveyor-type line.

The invention may also be used for testing properties and for qualityappraisal of powered materials that are used as active components inelectrodes of chemical power sources such as lithium batteries, supercapacitors, or as initial powdered components of solid electrolytes ofpower sources such as solid oxide fuel cell or thermal batteries.

The invention is based on the use of an electromagnetic method includingan algorithmic procedure for processing informative signals, and devicesfor the non-destructive testing of powered compositions of dielectricmaterials, such as cement and related materials based on cementincluding stucco, grouts, and the like.

The resulting data represent the electrical properties of the powderedmaterial. Electrical properties of powdered materials are determined onthe basis of the values for a set of parameters including quality factor(Q-factor), capacitance, dissipation factor, and dielectric permeabilityof the material. Using appropriate calibration, these parameter valuescan be related to such characteristics as moisture content, particlesize, and material composition.

The method and device allow determination and distinction betweenpowdered material samples of adequate quality for a specific intendedpurpose (“Good” samples) and samples inadequate for that same specificintended purpose (“Bad” samples). The sensitivity and specificity of thepresent invention in this regard has been demonstrated, as describebelow, by differentiating between powdered compositions comprised ofvarious different ratios of “Good” and “Bad” materials in varioussamples compositions, thus indicating gradations of quality.

By this approach, it was demonstrated that the present invention wascapable of distinguishing the differences between samples with variousquantities of impurities or unwanted reaction products, and to determinethe extent of sample degradation or ageing.

Along with the development of the electronic and electromechanicaldevice components and the software program for processing sensor signalinformation, a method for calibration was devised and applied for eachtype of powdered composition to be tested. This calibration routine isbased on data points obtained from known “good” and “bad” samples of agiven initial composition. The calibration data is provided to thesignal processing program and is used by the program for interpretingthe sensor signals obtained on “unknown” samples of the same type andintended composition. This allows for simplified measurements that arequite reliable.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, the quality of powered materialssuch as various cement compositions (referred to simply as “cement”below) is determined, indirectly, by measuring the values of a set ofelectro-physical parameters through the use of an electromagneticelectrocapacitance method. The primary transducer (sensor) is acapacitor which may be designed in, various forms, e.g. as two parallelplates, a hollow cylinder or as a coplanar capacitor. (The terms“transducer” and “sensor” will both be used in this application to referto the components of a resonant circuit that are altered in theircharacteristics by direct contact with the powdered material beingtested.)

Whatever the design of the capacitor, the entire surfaces of theelectrodes of the primary capacitance sensor (transducer) come intocontact with the powered material to be tested. Electrodes of thecapacitor can be made of a suitable material such as copper foil.

One of the versions of the capacitor design can be as follows. A copperfoil is glued to the interior surface of parallel plates of thedielectric transducer and is covered with varnish. In the case of acylindrical capacitor, the copper foil is glued to the interior of thedielectric cylinder. The sensitive elements of the plate capacitor canbe different by design.

Quality testing and control of powered materials may be performedmanually, automatically, or in-line directly on the manufacturing line.Using the present invention, testing can be done by a powdermanufacturer prior to shipment and by the powdered material end-userbefore final mixing and application of the material.

After initiation of the measurement, statistical data are automaticallycollected and summarized. According to the analysis program, the averagenumerical value of quality of the powder is determined by computation ofsensor data compared with standards data.

These data are stored in memory and displayed to the operator onsuitable data display device (in this case, a liquid crystal display).

In accordance with the intended modes of operation of the invention,measurements are to be performed at various points, or in variousregions, within the bulk powdered material mass. The data points thusobtained are integrated or averaged in the device. Each measurement isshown on the monitor, as well as the calculated average of theaccumulated total number of measurements. The integrated output orsummary of the data is displayed in the form of an indication as to theoverall status or quality of the powdered material being tested.

The electromagnetic capacitance method of powder quality controlinvolves placing the powder within the electric field of a capacitor andthe determination of its properties based on the corresponding responseof the electric field source. Powdered materials such as dry cements andcement compositions are essentially dielectrics. Therefore, the methodemployed provides for measurement of the dielectric properties of thematerials. Data on the electromagnetic (dielectric) properties comprisethe initial information for determining quality of the materials tested.

Dielectrics like cement have mainly bound charges, which form dipoles.Contained within powdered compositions of this kind there are various(sometimes unwanted) components such as moisture or acids. There arealways free electric charges in these compositions, which createelectric conductivity. Thus, in electric fields of powder-likecomposites there are currents caused by polarization, and conductivitycurrents. Overall, electrical currents found in such a system arise fromvarious sources and are complex.

Various types of polarizations are known. For the invention presentedhere, consideration not only data on electric currents, but also of dataon structural polarization is important. Structural polarization is anadditional part of the mechanism that influences the relaxation. Inparticular, structural polarization is observed in moisture absorbingmaterials. Cement composites are active absorbers of moisture. Inaddition, they contain components that are acidic or readily react toform acids in the presence of water or carbon dioxide.

Cement-based composition quality depends, to a great extent, on theamount of moisture absorbed, the presence of acidic compounds, and thedeviation of the percentages of components from their intended orstandard proportions in the mixture. In an electromagnetic field, thetotal current density in powdered materials equals the sum of theconductivity current and polarization current densities.

When the effects of electromagnetic fields the total density of currentsin powdered materials equals the amount of density of conductivitycurrent and currents of shift. The following equation can be written fortotal density of currents:

$\begin{matrix}{{\overset{.}{J} = {\underset{{cond}.}{\overset{.}{J}} + \underset{{displ}.{moment}}{\overset{.}{J}} + \underset{{displ}.{polar}.}{\overset{.}{J}}}},} & (1)\end{matrix}$

Where:

-   -   J_(cond.) Is the conductivity current density?    -   J_(displ.moment.) is the capacitance displacement current        density,    -   J_(displ.polar.) is the density of polarization displacement        current.

The density of the capacitance current and density of polarizationcurrent depend on the frequency of the electric field used to drive thetransducer.

Equation (2) shows that component values are dependent on the frequency.

$\begin{matrix}{{{{J_{{displ}.{moment}} + J_{{displ}.{polar}}} = {{{j\omega}\; ɛ_{0}ɛ_{\infty}^{\prime}E} + {{j\omega ɛ}_{0}{\sum\frac{\Delta \; ɛ_{i}}{1 + {j\; \omega \; \tau_{i}}}}}}};}{where}{\omega = {2\pi \; {f.}}}} & (2)\end{matrix}$

Due to losses caused by friction of polarized particles, polarizedcurrent surpasses (exceeds) the voltage of the capacitor electric fieldat angles less than 90°. Therefore polarized current needs to be dividedinto two parts (components), which correspond, accordingly, toconductivity current and capacitance current. For that reason, totalcurrent density and its active and reactive components, are consideredin the analysis of the present invention.

From the above discussion, let us consider how complex dielectricpermeability depends upon complex conductivity:

$\begin{matrix}{{\overset{\sim}{ɛ} = {{ɛ^{\prime} - {jɛ}^{''}} = \frac{\overset{\sim}{\sigma}}{{j\omega ɛ}_{0}}}},} & (3)\end{matrix}$

and accordingly, the dependence between the real and imaginarycomponents is:

σ′=ω∈₀∈″ and σ″=ω∈₀∈′  (4)

Material dielectric permeability ∈′ characterizes the charge density onplates of the transducer, i.e. the degree of polarization and number ofdipoles. The number of dipoles depends upon the amount of foreignparticles.

The imaginary component ∈″, i.e. dielectric permeability, characterizeslosses related at a given frequency f in a powder-like material tested.Such losses are caused by both the increased conductivity and thefriction experienced by polarized particles of foreign matter.

In addition to the above informative parameters, it is convenient toconsider the dielectric losses tangent to the dielectric disposition:

$\begin{matrix}{{{{tg}\; \delta} = \frac{ɛ^{''}}{ɛ^{\prime}}};} & (5)\end{matrix}$

or as another parameter, the Q-factor of powder-like materials such ascement for building purposes.

$\begin{matrix}{{Q = {\frac{1}{{tg}\; \delta} = \frac{ɛ^{\prime}}{ɛ^{''}}}};} & (6)\end{matrix}$

The above theoretical analysis of the method of the present inventionsuggests that one should choose and optimize informative parameters asemployed in the invention. Accordingly and as set forth in claims 2, 4,5 and 6, below, the following should be done for each kind of materialtested:

-   -   Conduct research and choose an optimal frequency (f) for        measurements    -   Measure and analyze informative parameters:        -   ∈′ (or dielectric permeability, ∈),        -   tg δ, dielectric disposition        -   losses in terms of Q-factor Q, and    -   Select the test mode that utilizes the most effective parameters        for a given sample.

Results and conclusions from the research carried out to reduce topractice, develop and test the present invention are shown in theExamples provided below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the dielectric permeability (∈) as a function offrequency. As a test sample, the cement-based composition designated as“584” was used. The samples were given the following designations: 101(bad sample); 102 (good sample). Measurements were carried out with acylindrical primary transducer.

FIG. 2 illustrates the dissipation factor (tg δ) as a function offrequency. As a test sample, the cement-based composition 584 was used.The samples were given the following designations: 201 (bad sample); 202(good sample). Measurements were carried out with a cylindrical primarytransducer.

FIG. 3 illustrates the dielectric permeability (s) as a function offrequency. As a test sample, the cement-based composite 584 was used.The samples were given the following designations: 301 (bad sample); 302(good sample). Measurements were carried out with a primary transducermade of parallel flat electrodes.

FIG. 4 illustrates the dissipation factor (tg δ) as a function offrequency. As a test sample, the cement-based composition designated as584 was used. The samples were given the following designations: 401(bad sample); 402 (good sample). Measurements were carried out with aprimary transducer made of parallel flat electrodes.

FIG. 5 illustrates dielectric permeability as a function of frequency.As a test sample, the cement-based composition designated as “795” wasused. The samples were given the following designations: 501 (badsample); 502 (good sample). Measurements were carried out with a primarytransducer made of parallel flat electrodes.

FIG. 6 illustrates dissipation factor (tg δ) as a function of frequency.As a test sample, the cement-based composition 795 was used. The sampleswere given the following designations: 601 (bad sample); 602 (goodsample). Measurements were carried out with a primary transducer made ofparallel flat electrodes.

FIG. 7 shows the image of the primary capacitive transducer designatedas “EC-5C”.

FIG. 8 shows the image the primary capacitive transducer designated“EC-7C”.

FIG. 9 illustrates dependence of the relative change in the Q-factor asa function of frequency. As a test sample, the cement-based composition795 was used. The samples were given the following designations: 901(bad sample); 902 (good sample). Measurements were carried out on acapacitive primary transducer designated as EC-5C.

FIG. 10 illustrates dependence of relative change of material Q-factoras a function of frequency. As a test sample, the cement-basedcomposition 795 was used. The samples were given the followingdesignations 1001 (bad sample); 1002 (good sample). Measurements werecarried out on a capacitive primary transducer EC-7C.

FIG. 11 illustrates results of non-destructive testing of “good” and“bad” samples of cement-based composition 795. The frequency of themeasurement for 1101 was f=74.5 kHz; for 1104, it was f=90 kHz. 1102,1103 show the charts are approximated by polynomials of the seconddegree with correlation coefficients as follows: for 1102, R²=0.9983 andfor 1103, R²=0.9754. Measurements were carried out with the capacitiveprimary transducer EC-7C.

FIG. 12 shows results of testing “good” and “bad” samples of acement-based composition after it has been stored for 50 days 1201,1202, and 1203 are “bad” Samples, and 1204, 1205, and 1206 are “good”samples; The frequency of the measurement was 70 MHz.

FIG. 13 shows results of evaluation of the cement composition 797samples with different levels of the ageing. Sample 1301 was fresh,samples 1302 was storage for one month, and sample 1303 was exposed tosevere aging conditions.

FIG. 14 shows results of testing of cement-based composition 941 sampleswith different level of acid content: 1401 was a “good” samples, 1402had a 0.37% citric acid, 1403 had a 0.75% citric acid content, and 1403was designated as a “bad” sample.

FIG. 15 shows the image of a equivalent measuring circuit: 1501 is thesignal generator for the AC signal used; 1502 is the inductance coil L;1503 is the resistor r_(L) representing the resistive loss in theinductance coil; 1504 is the device for Q-factor measurement; 1505 is avariable capacitance; 1506 is the self-capacitance C₀ of the workingsensor; 1507 is the added capacitance C_(d) of the filled working sensordue to the effect of the of dielectric permittivity of the dry powder;and 1508 is the resistor r_(d) representing the dielectric losses in thedry powder.

FIG. 16. shows a diagram of electrical equivalent circuit of the workingsensor filled with powder. 1601 is the resistor r_(d) representingdielectric losses in the dry powder, 1602 is the self-capacitance C₀ ofthe working sensor, 1603 is the added capacitance C_(d) of the filledworking sensor due to influence of dielectric permittivity of drypowder, 1604 is the added capacitance C_(h) of filled working sensor dueto influence of the moisture content of the powder, 1605 is the resistorr_(h) representing the effect of the moisture content of the powder.

FIG. 17. is a block diagram of the measuring device wherein 1701 is thepowder reservoir, 1702 is the powder, 1703 is the sensor case, 1704 arepotential measuring electrodes, 1705 is the electromagnetic acoustictransducer, 1706 is the powder level sensor, 1707 is the capacitance ofthe powder level sensor measuring unit, 1708 is the control unit, 1709is the pulse generator, 1710 is the signal generator, self-capacitanceand Q-factor measuring unit, 1711 is the phase measuring unit, 1712 isthe frequency measuring unit, 1713 is the circuit that generates theinformative signal, 1714 is the capacitance of the “filled sensor”determination unit, 1715 is the classification unit, and 1716 is thecircuitry for determining the correction signal required.

DETAILED DESCRIPTION OF THE INVENTION

Prior to testing to determine the quality of powdered materials,laboratory standard samples of the materials must be prepared. Forexample, standards samples of cement-based compositions of good quality,as well as samples containing component quantities that deviate from thestandards, are required. The components chosen for testing have to bethose that determine the quality of materials, that is, those thatsignificantly affect the quality of the materials tested. The totalnumber of samples for each type of material to be tested should no lessthan nine. These sample data are used for construction of qualitydiagrams, look-up tables, or calibration curves to be used incalibrating and interpreting the data obtained from test measurements.

Samples of materials are tested following the method of non-destructivequality control described in the present invention. The quality of thematerial is determined on the basis and the results of measurements, ascompared with the collected calibration data.

Just prior to testing, a powder such as cement is placed on a specialpallet or is tested directly in its transport container. To obtain acomprehensive quality evaluation, cement samples are taken fromdifferent depths in the container or the bulk material. Before beingtested the powdered materials are well mixed in the transport container.

The capacitance primary multi-sectional transducer, in general, maycontain a number of alternating electrodes. Some of the electrodes areat potential, and others are grounded. On both sides of each electrodeat potential there are grounded electrodes of identical forms. Thisallows locating the electric field of each potential electrode withinthe range of its adjacent sections. An adjustable frequency signalgenerator is used to drive the sensor.

Prior to testing, measuring is made, without powdered materials, of thefollowing:

-   -   sensor capacitance—C₀ and    -   Q-factor of the resonance circuit, Q₀, with the capacitance        sensor being a component of the (self) resonant system.

The value of Q-factor of resonance circuit is described by equation (7):

$\begin{matrix}{{Q_{0} = \frac{L}{C_{0}r_{L}}},} & (7)\end{matrix}$

where:

L is the circuit inductance (FIG. 15), C₀ is the capacitance sensorself-capacitance, r_(L) is the inductive impedance of the coil. orresistance of real losses of inductance coil).

After measures of Co and Qo are made, the transducer is placed on thesurface of the powdered material to be tested. Then an impulse generatoris turned on. The generator supplies power to a coil of a vibratingelectromagnetic acoustic transducer, which is assembled (joined) with acapacitance sensor as one equipment unit. The vibrating electromagneticacoustic transducer has a constant magnet, a flat coil and a coil casemade of polyurethane. Due to interaction of the magnetic field and thecoil field, a Lorentz effect occurs. The Lorentz effect causes thecapacitance sensor to vibrate at the ultrasound frequency. Then thesensor is lowered inside the powder-like material at a certain depthdetermined by an auxiliary capacitance sensor. The coplanar plates ofthe auxiliary capacitance sensor, in the form of thin band stripes, arelocated on the interior of the case for the working capacitance sensor,over its lower working end.

When the working volume of the transducer is filled up with powder, thecapacitance of the auxiliary sensor will reach a certain predetermined(threshold) level. Exceeding this level of the capacitance serves togenerate a signal that stops the vibration. Then the values of phase andfrequency shifts in the sensor signals are recorded, which mean that thesensor is full of powder (has been fully plunged into powderedmaterial). Vertical openings in the upper parts of the plates of theworking capacitance sensor allow uniform powder density in each sensorcapacitance area.

The Q-factor of the capacitive transducer which immersed in the powderedmaterial is determined by equation (8):

$\begin{matrix}{{Q_{1} = \frac{L}{( {C_{0} + C_{d}} )( {r_{L} + r_{d}} )}},} & (8)\end{matrix}$

where:

-   -   C_(d) is the added capacitance of the sensor, which is caused by        the influence on the field of the transducer and the dielectric        permeability of the powder tested. (FIG. 15)    -   r_(d) is the resistance or dielectric losses in a tested powder.

The relative change of sensor Q-factor can be expressed on the basis ofequations (7) and (8) as:

$\begin{matrix}{Q_{r} = {\frac{\Delta \; Q}{Q_{0}} = {\frac{{C_{0}r_{d}} + {C_{d}( {r_{L} + r_{d}} )}}{( {C_{0} + C_{d}} )( {r_{L} + r_{d}} )}.}}} & (9)\end{matrix}$

In the case when Cd is lower, the sensor self-capacitance Co and r_(d)and r_(L) are of the same order such as in the case of dry powder cement(see examples below where these conditions are present), Q_(r) isexpressed as:

$\begin{matrix}{{Q_{r} = \frac{r_{d}}{r_{L} + r_{d}}},} & (10)\end{matrix}$

Since r_(L) is a constant value, Q_(K) is determined by the value ofdielectric losses of dry powder (r_(d)). This is easily supported byexperimental dependency of relative Q-factor on frequency for samples ofcement-based powders of different quality as shown below.

The present invention provides a method of quality control for powderedmaterials, which are normally stored in an ordinary manufacturingenvironment. In such situations, it is always important to take intoconsideration the effect of humidity. Increasing moisture incement-based materials leads to increased sensor capacitance due to thehigh dielectric permeability of water. Therefore, the Q-factor of thetransducer is lowered because of the greater amount of moisture andstraightforward conductivity on the surface of moistened grains of thepowdered materials.

An equivalent scheme for a capacitive sensor for this particular case ispresented in FIG. 16. Sensor impedance is computed by the followingequation (11)

$\begin{matrix}{{Z = \frac{( {r_{d} + \frac{1}{j\; \varpi \; C}} ) \cdot r_{h}}{r_{d} + \frac{1}{j\; \varpi \; C} + r_{h}}},} & (11)\end{matrix}$

where:

-   -   C=C₀+C_(d)+C_(h),    -   C_(h) describes the increase in sensor capacitance caused by        moisture effect,    -   r_(h) is the conductivity of powdered material due to moisture.

If we separate the real and imaginary parts in Equation 5 we will obtainthe following:

$\begin{matrix}{Z = {\frac{r_{h}\lbrack {1 + {\varpi^{2}C^{2}{r_{d}( {r_{d} + r_{h}} )}}} \rbrack}{1 + {\varpi^{2}{C^{2}( {r_{d} + r_{h}} )}^{2\;}}} - {j\; \frac{\varpi \; {Cr}_{h}^{2}}{1 + {\varpi^{2}{C^{2}( {r_{d} + r_{h}} )}^{2}}}}}} & (12)\end{matrix}$

Dividing the real component of complex impedance by the imaginarycomponent we will obtain cotangent of the phase angle:

$\begin{matrix}{{{{ctg}\; \phi}} = {\frac{1 + {\varpi^{2}C^{2}{r_{d}( {r_{d} + r_{h}} )}}}{\varpi \; {Cr}_{h}}.}} & (13)\end{matrix}$

Let us consider the value of product A=ωCr_(h)=ω(C₀+C_(d)+C_(h))r_(h).The frequency of measurements ω, capacitance C₀ and C_(d) are constantvalues. Capacitance C_(h) increases when moisture rises. Resistancer_(h) decreases with increased moisture. Within certain values ofmoisture, the value of the product residual is constant. Beyond thelimits of those values, the A value can be graded on the basis ofcondenser capacitance of the condenser (transducer) filled with powder.

Condenser capacitance of the condenser filled with powder is determinedafter testing has been made on the data on the basis of frequencyapplying Thompson's equation. In this way the value of correctionalsignal A can be obtained.

When the term A is used in equation (13) we will get the following:

$\begin{matrix}{{{{ctg}\; \phi}} = {\frac{1 + {\varpi^{2}C^{2}r_{d}^{2}} + {A\; \varpi \; {Cr}_{d}}}{A}.}} & (14)\end{matrix}$

In equation (14) each value, except r_(d), is obtained by measurement.The value of the resistance, therefore, can be computed from equation(14). This resistance describes dielectric losses on grains of powderedmaterials and is a part of the informative parameter Q signal. By thevalue of such an informative signal, and on the basis of data from thecalibration curve or look-up table, we can obtain values that describequality of powdered material tested.

If we take into account that the resistance r_(h), under conditions oflow humidity, significantly exceeds r_(d) then the expression (8) can besimplified to the following:

$\begin{matrix}{{{{ctg}\; \phi}} = {\frac{1 + {{r_{d} \cdot A}\; \varpi \; C}}{A}.}} & (15)\end{matrix}$

After the first measurement, the sensor comes up to the surface of thepowdered material. Then vibration is turned on for cleaning ofelectrodes of the sensor of residual powder. Sometimes powder may clingon the electrode surface. For thorough electrode cleaning, the vibrationcan require several more seconds. The cleaning process is regulatedaccording to Q-factor values of the resonance circuit, together with thecapacitive sensor.

When Q-factor approaches the predetermined level or threshold, vibrationstops.

This threshold value of the Q-factor of the resonance circuit, togetherwith the capacitive sensor is set as equal to its unloaded Q-factor. Thevalue of self Q-factor is set at the moment before first “plunging” ofthe sensor into powdered material. At the same moment an adjustment canbe made, which depends upon the physical properties of the powderedmaterial. Such an adjustment is chosen within 5% of resonance circuitQ-factor; it varies because of powder properties. Thus, the thresholdvalue equals the unloaded (no powder) circuit Q-factor minus anadjustment value.

After capacitive electrode cleaning, repeated measurements are carriedout in various areas of powdered material in the same container. Thenaverage values are computed. The total number of measurements should beno less than nine. The quality of powdered material is evaluated by theaveraged value of informative signals obtained from all measurements. Anoperational diagram of the equipment for control of powdered materialquality is presented in FIG. 17.

The equipment consists of a sectional capacitive sensor of periodicstructure with alternating “at potential” and grounded electrodes, avibration electromagnetic acoustic transducer made as one unit with acapacitive sensor and mounted in the same case, and an auxiliarycapacitive sensor with coplanar electrodes placed on side walls of thecase of the working capacitive sensor.

An inductance coil together with the working capacitive sensor makes aresonance circuit. This resonance circuit is connected to a signalgenerator which is combined with a diaphragm for measuring sensor selfcapacitance and Q-factor of the resonant circuit in a single measuringdevice.

A generator driving the vibrating electromagnetic acoustic transducertransmits impulses to a flat inductance coil mounted in a polyurethanecase. The circuit for measuring capacitance of an auxiliary capacitivesensor transmits command signals to the impulse generator diaphragm tostop vibration, and also to the circuits for measuring phase andfrequency to record values of phase and frequency shifts, whichcorrespond to a fully inserted or plunged sensor. The equipment also hasa circuit for measuring the capacitance of the working sensor, which isfully submerged in powdered material, a circuit for processing incomingsignals, a circuit for generating the informative signal, and a circuitfor evaluating and indicating the quality of the of powdered materialbeing tested.

EXAMPLES

The Examples described below are provided for illustration purposes onlyand are not intended to limit the scope of the invention.

Example 1

The cylindrical capacitive transducer has copper electrodes that are 70mm×30 mm with a thickness of 10-20 μm. The electrodes are made ashalf-cylinders and mounted inside the lower part of the transducer casemade of dielectric in a cylinder-like form. The diameter of thedielectric is 60 mm and the height is 70 mm. The thickness of the wallin the areas of electrodes is 1.00 mm. As an auxiliary induction for thecircuit, a coil with a diameter of 18 mm, together with two wire coilswith an outside diameter of 0.84 mm was connected to it is used.

Since the height of the entire cylindrical part of the capacitivetransducer is commensurable with the entire height of the transducer L₁and compatible with the height of the copper electrode L₂ thecapacitance is described by the expression (16):

$\begin{matrix}{{C = {\frac{ɛ_{0}ɛ_{r}L_{2}}{\pi}\ln \; \frac{{\sin ( \frac{\varphi_{3} - \varphi_{0}}{2} )}{\sin ( \frac{\varphi_{2} - \varphi_{1}}{2} )}}{{\sin ( \frac{\varphi_{2} - \varphi_{0}}{2} )}{\sin ( \frac{\varphi_{3} - \varphi_{1}}{2} )}}}},} & (16)\end{matrix}$

where:

-   -   ∈₀=8.854×10⁻¹²,    -   ∈_(r)=∈′=1,    -   Φ are the values of angles of electrode segment in a cylindrical        co-ordinate system.

Capacitance values can be expressed in numbers as: C=1.81 pF.

The research described in Example 1 was conducted on the basis of twocement compositions: 584 and 795. There were two samples for eachcomposition as described above, which were labeled as “good” and “bad”.A Q—meter (HP) instrument and above described transducers were used formeasurements. For capacitive transducers with capacitance valuescomputed by equation (1), and measurements of the Q-factor andcapacitance were made over the frequency range of 12-70 MHz. On thebasis of the measurements of capacitance and Q-factor, dielectricdissipation and dielectric permeability values were computed.

Table 1 describes the following parameters:

-   -   f=frequency in MHz    -   Q₀=Q-factor prior to placing a tested cement powder into the        transducer    -   C₀=capacitance prior to placing a tested sample into the        transducer    -   Q₁=Q-factor after placing a tested sample into the transducer    -   C₁=capacitance after placing a tested sample into the transducer    -   ∈×10⁻³ dielectric permeability of the tested powder    -   tg δ×10 ⁻³—dielectric dissipation of the sample being tested.

TABLE 1 Effect of electric field frequency on capacitance values,Q-factor, and computed values of dielectric permeability and dissipationfactor for cement-based composite 584. No F Q₀ C₀ Q₁ C₁ ∈ × 10⁻³ tgδ ×10⁻³ 1 12 93 420.5 91 419.5 2.37 99.37 2 15 100 268 99 267 3.73 27.07 320 113 148 110 146 13.5 17.86 4 25 123 91.4 118 90 15.3 22.49 5 35 14443.5 132 42 34.48 18.3 6 45 160 22.8 139 21.9 39.5 13.46 7 70 231 21.5201 20.5 46.5 13.89 8 12 91 421.5 90 420 3.56 34.31 9 15 100 268 98 26810 20 113 148 109 147 6.76 48.06 11 25 123 91.5 117 90 16.4 25.43 12 35143 43.5 134 42 34.48 13.62 13 45 159 22.9 142 21.9 43.67 17.24 14 70231 21.5 194 20.5 60.46 17.75

Numbers 1-7 correspond to the “good” sample and number 8-14 correspondto the “bad” sample.

Values for the parameters ∈ and tg δ are computed by equations:

$\begin{matrix}{{ɛ = \frac{C_{0} - C_{1}}{C_{1}}};} & (17) \\{{{tg}\; \delta} = {\frac{( {Q_{0} - Q_{1}} )C_{0}}{Q_{0}{Q_{1}( {C_{0} - C_{1}} )}}.}} & (18)\end{matrix}$

FIGS. 1 and 2 illustrate the measured dielectric permeability anddissipation factor of a tested powder as a function electric fieldfrequency. Parameter values in Table 1 and diagrams 101 and 102 in FIG.1 suggest that cylindrical transducers are better used in the frequencyrange above 55 MHz. In the case of lower frequencies, the parameter ofdielectric permeability does not provide adequate information forquality control of certain samples.

Analysis of data on dielectric dissipation from Table 1 and diagrams 201and 202 in FIG. 2 shows that the transducer design in question cannot beused for control using the dielectric dissipation parameter (tg δ) inall frequency ranges. Differences between the obtained values of thisparameter are within the method error range.

Example 2

A capacitive transducer is made of two parallel electrodes of copperfoil of size 120 mm×30 mm with a thickness of 20 microns. Electrodes areplaced inside the lower part of dielectric transducer case. The distancebetween the electrodes is 4 mm. The thickness of dielectric wall in thearea of electrodes is 1 mm. In the case when the total area of condenserparallel plates is large, such that the electric field distorting effectcan be ignored on the plate edges, Gauss's law was applied:

C=∈ ₀ ∈S/d=8.854×10⁻¹² ×∈′S/d=7.969 pF,  (19)

In this case, transducer capacitance is higher as compared with thecylindrical transducers in Example 1. The increased capacitance ascompared with Example 1 allows increased sensitivity. Measurements weremade by the instrument described in Example 1. The same samples ofcement-based compositions were used.

The results of measurements are presented in Table 2. On the basis ofthese data diagrams were drawn (FIG. 3-6), which illustrates thedependence of the measured and computed parameters upon frequency.

In Table 2: samples 1-4 and 9-12 were “good” cement-based compositions584 and 795 and samples 5-8 and 13-16 were “bad” cement-basedcompositions 584, 795

Analysis of data on parameters in Table 2 and diagrams in FIGS. 3-6suggests that it is better to use a capacitive transducer with parallelflat electrodes, and not consider the dielectric dissipation parameter,but look at the relative change of Q-factor, which can be computed bythe following equation:

$\begin{matrix}{Q_{relative} = {\frac{Q_{0} - Q_{1}}{Q_{0}}.}} & (20)\end{matrix}$

TABLE 2 Measured capacitance values, Q-factor, and computed values ofdielectric permeability and dissipation factor of the test powder as afunction of frequency in the range 30-60 MHz. F No MHz Q₀ C₀ Q₁ C₁ ∈10⁻³tgδ10⁻³ 1 584, good 30 111 273.02 97 266.7 23.15 56.2 2 584, good 40 135147.5 102 139.8 52.2 45.9 3 584, good 50 161 88.3 88 78.1 115.5 44.6 4584, good 60 180 53.4 60 37.7 294 37.8 5 584 bad 30 111 272.45 96.3265.5 25.5 53.9 6 584 bad 40 135 147.42 98 137.6 66.6 41.98 7 584 bad 50160 87.93 82.8 74.55 152.2 38.3 8 584 bad 60 179 53.25 50 30.18 433.233.26 9 795 good 30 110.5 272.6 102 265.8 24.9 28.58 10 795 good 40 135147.15 112 138.98 55.5 27.4 11 795 good 50 159.5 88.26 106 75.68 142.522.2 12 795 good 60 178 53.4 70 33.96 364 23.8 13 795 bad 30 110 271.7192.5 265.72 22 78 14 795 bad 40 134.5 146.7 91 138.75 54.2 68.4 15 795bad 50 158 87.72 74 76.26 130.6 54.99 16 795 bad 60 175 52.9 45 34.41349.5 47.2

Analysis of numbers in the above Tables 1 and 2 and diagrams shows thatuse of a capacitive transducer with parallel flat electrodes of the sizedescribed in Example 2 is problematic if an informative parameter ofdielectric permeability is considered.

When a capacitive transducer with parallel flat electrodes was used itwas found out that the small distance between electrodes makes itdifficult to evenly fill the space between electrodes with powder. Andit is also difficult to entirely remove the powder.

Example 3

In this example, results of powder testing using transducer EC-5C areshown (FIG. 7). The transducer has parallel flat electrodes of copperfoil. Electrodes sizes are: a=100 mm, b=12.5 mm and thickness=15microns. Electrodes are mounted inside the lower part of a transducercase. The case is made of dielectric. The thickness of the wall in thearea of the electrodes is 1 mm. The distance between electrodes L is 7mm. The increased distance between electrodes is needed to ensure evenfilling of the space between electrodes when a transducer is insertedinto the powder being tested.

To compute capacitance of a capacitor with increased space betweenelectrodes equations (6 and 7) were applied. The equation takes intoaccount the distortion of the field at the capacitor edges caused by theelectrodes plates.

$\begin{matrix}{{C = {{4\; ɛ\; a\; \frac{b}{l}} + {0.5\; C_{0}}}},} & (21) \\{{where};{C_{0} \cong {\frac{16\; ɛ\; a}{{\frac{a}{b}{arcsh}\; \frac{b}{a}} + {{arcsh}\; \frac{a}{b}}}.}}} & (22)\end{matrix}$

The resulting capacitance value is C=2.45 pF.

An experimental measurement of the transducer capacitance over thefrequency range 24-70 MHz produced and average value C=2.15 pF with atotal of 20 measurements. This proves that the formula is acceptable forcomputational purposes.

The transducer is installed on a stand that allows strictly verticalmovement while the transducer is being inserted into the test powdermaterial. In the measurement process, the space between electrodes isentirely filled with the same quantity of cement mixture. In Table 3 theresults of Q-factor are shown after seven measurements of composition795 at each frequency.

Calculations indicate that error in measurements is no more than 5-7%.To evaluate in quantitative terms the degree of powder quality deviationfrom standard, a higher sensitivity of the method employed is needed.These procedures are described below.

TABLE 3 Q-factor as a function of frequency and results of replicatemeasurements. F Serial number of a measurement # MHz 1 2 3 4 5 6 7Average 1 30 49.5 49.5 49.2 51.9 75.0 53.9 54.7 54.8 2 40 100 104.8103.2 103.9 121.0 111.6 105.7 107.2 3 50 178.9 180 176.5 177.2 197.3184.2 178.9 181.9 4 60 272.2 269.7 269.2 273.9 300.9 285.7 265.9 276.8 570 384.2 393.6 382.2 392 401.7 406.3 394.6 393.5 6 30 107.6 108.9 9999.5 107.1 104 110 105.2 7 40 204.0 201.6 194 193 204.2 187 204.9 198.48 50 315.6 320 284.8 294.9 317.2 300.7 314.3 306.8 9 60 441.3 466.7413.9 424.7 451.9 431.8 454.3 440.7 10 70 571.8 574.5 567.4 563.3 592.0594.9 592.4 579.5

The test powder is the cement-based composition 795. The powder beingtested is placed between the electrodes of the capacitor.

Example 4

Higher sensitivity of the method employed is attained thru increasingthe capacitance of the transducer. Measurements were carried out by theuse of transducer EC-7C (FIG. 8). The sizes of flat parallel copperelectrodes were a=100 mm, b=30 mm and the thickness was 15 micron. Thetransducer capacitance computed according to equation (6) is C=5.01 pF.

Q-factor and the losses of the Q-factor (Q_(relative)) were accepted asan informative parameter in the process of testing of powdered materialswhen powder was placed in the space between the electrodes. Computationswere made according to equation (5).

TABLE 4 Effect of frequency and of enlarged transducer capacitance onQ-Factor losses when the test powder was placed in the space between theelectrodes of a capacitive transducer. # F MHz Q₀ C₀ Q₁ C₁ Q_(relative)# 795_((EC-5C)) 1 24 87 453.3 78 441.9 103.4 2 30 100 287 82.9 275.6 1713 40 120 155.7 81.4 142.4 321.7 4 50 143 94.2 67.5 76.5 528 5 60 16358.5 43.2 31.35 735 6 24 86.6 452.4 79.5 441.9 81 7 30 99.1 287 87.1275.3 120.1 8 40 119.8 155.7 90.8 142.4 242.1 9 50 141.5 94.2 77.8 76.45450.2 10 60 160 58.7 56.2 32.1 648.7 #795_((EC-7C)) 11 24 87.1 451.085.2 441.0 22.5 12 30 98.3 287.2 95 277.1 34.5 13 40 112.1 155.8 104.5144.7 72.6 14 50 116.6 94.4 104 81 121.6 15 60 106.4 59.2 89 41.9 195.416 70 87.2 27.1 67.3 21.3 296.2 17 24 81.9 452.2 77.5 441.9 56.2 18 3089.3 286.7 82 275.5 89.1 19 40 90.38 155.7 77.3 142.2 169.3 20 50 78.7494.3 62 76.3 270 21 60 53.59 59.0 38.7 31.6 385 22 70 32.92 31.0 21.816.4 510

For the powder designated 795 transducers EC-5C and EC-7C were used.Data are presented in Table 4, No. 1-10 are from EC-5C and in Table 4,No. 11-18 are from EC-7C.

To find out if the test procedure can be performed over a widerfrequency range, we also made measurements at low frequency range up to100 KHz. Results of the measurements are presented in Table 5.

TABLE 5 Q_(average) for samples 795 with different ratios betweenquantity of the “Good” and “Bad” materials. Percentage of mixture BADG/B G/B G/B GOOD # 100 25:75 50:50 75:25 100 Note 74.5 kHz 1 11 11.5 1516.5 23.5 2 11 11.5 13.5 16 23.5 3 10 12.5 14.5 16 24 4 11 11.5 14 16.325 5 11 12 14.7 17 23.5 6 11 12 14.2 16 24 10.83 11.83 14.32 16.3 23.92Average 90.0 kHz 1 13 16 19 24 30.2 2 14.3 16 19.5 22.9 30 3 14.3 1419.5 24 30 4 13 14 17 22 30.5 5 14 15.5 19.5 23 28 6 14 14.8 18.6 2229.8 13.77 15.05 18.85 22.98 29.75 Average

Table 5 and FIG. 11 contain results of evaluation of sample 795,including computed average values. Measurements were made at twofrequencies; 74.5 kHz and 90 kHz. The graphs are approximated bypolynomials of the second degree with correlation coefficient R²=0.9983and R²=0.9754. One can notice precise recurrence of dependencies at bothfrequencies.

The results of the evaluation are presented in FIG. 11 and Table 5 andconfirm that the method and transducers developed in accordance with theinvention allow identification, with high level of accuracy, “Good” and“Bad” samples, as well as compositions comprised of various proportionsof the original “Good” and “Bad” samples.

On the basis of the comparative analysis of data in FIGS. 9-11, one canconclude that a capacitive transducer with large electrode areas and aplatform surface covered with copper foil such as EC-7C has a highersensitivity and allows better determination of differences among samplesof powdered materials with various properties. And this sensitivity isdemonstrated at each frequency level.

Example 5

The research that had been conducted earlier also gave the followingresult. Within a 50 days period of measuring parameters of “good” and“bad”, the sample characteristics tended to become similar. This may becaused by aging of the “good” samples. Results of comparative evaluationof “Good” and “Bad” samples, which have been stored for a period of 50days, are presented in FIG. 12 and Table 6. At the beginning of themeasurements the difference in the value of a quantity measurementparameters for the fresh “Good” and “Bad” samples was approximately 50relative units at a frequency of 70 MHz.

TABLE 6 Results of comparative evaluation of “Good” and “Bad” samples ofcement composition, which have been stored for a period of 50 days.Time, days Quality 10 20 30 40 50 Bad 20.2 1.5 1.1 1.0 0.5 Bad 8.2 3.01.4 0.5 0.0 Bad 10.0 8.0 0.9 1.0 2.0 Good 18.0 15.0 1.5 2.0 3.1 Good46.2 21.5 3.1 3.0 1.0 Good 48.7 20.8 −5.3 3.0 1.0

The data in these Examples show the usefulness of the method andequipment of the present invention. For instance, the recommended methodand instruments allow monitoring the aging of cement-based compositesand ensure more precise control of powdered material quality in theprocess of manufacturing and storage. This is particularly important forquality control of construction materials.

Below results of the non-destructive evaluation of composition 797 withdifferent level of ageing are presented. This testing was carried outdone on composition 797 based on cement that had been aged as follows:

-   -   1. Fresh    -   2. Aged #1 storage for a period of one month    -   3. Aged #2 (exposed to severe conditions.        Results of testing samples 797 are presented in Table 7 and FIG.        13.

TABLE 7 Numerical values of informative parameter Q. 90 kHz Fresh Aged 1Aged 2 Qrelative 81 70 70 80 77 76 78 74 71.5 82 81 80 80 75 72 81 80 77Average: 80.33 76.17 74.42

In FIG. 13, a smooth change of an informative signal Q_(relative) as afunction of the degree of ageing of the test cement is shown. Results ofevaluation presented in FIG. 12, Table 6 and FIG. 13, and Table 7indicate that the method and device that comprise the present inventionallow determination of the level of powdered sample ageing.

Example 6

In this Example, results of the non-destructive evaluation ofcompositions with different citric acid content are described. Thistesting was done with samples based on the composition 941 withdesignations and citric acid content as follows:

-   -   1.941 Good    -   2. 941 Bad    -   3. 941+0.75% citric acid    -   4. 941+0.37% citric acid

In FIG. 14 the change of an informative signal as a function of thepercent of the acid in the 941 cement-based composition is shown. Fromthe analysis of the graph in FIG. 14, it is clear that the method allowsunequivocal distinction between 941 “Good” samples and “Bad” samples.

Example 7

Powder residue on the electrodes after measurement causes some errorwhen Qo is measured. To determine effect of powder level in between theelectrodes on the error of Qo value, a separate investigation wasconducted.

Measurements while filling the space between the electrodes to differentquantity of the powder were carried out. Various powder quantities wereused while the space between the electrodes was being filled. In Table8, data on measuring Q-factor are shown for the cases when powdercontent, prior to measuring Qo value, was 70% and 30% of maximumcontent. It was noticed that the error is higher when the residualincreased. Results are presented in the Table 8.

TABLE 8 Effect of powder content in the space between electrodes on theerror in the Q-factor. Q Measurement error (%) Sensor Sensor SensorSensor Sensor F without with 70% with 30% with 70% with 30% # MHz powderof powder of powder of powder of powder 1 24 87 81.3 84.7 6.6 2.6 2 30100 84 89.2 16.0 10.8 3 40 120 90 105 25.0 12.5 4 50 143 108 127.3 24.410.97 5 60 163 79.8 134.5 51.04 17.48

As the data in Table 8 show, residual powder can significantly affectthe results of measurements.

Example 8

In the transducer design, an emitter of low frequency ultrasound(longitudinal oscillation) was used to provide vibration. Transmittingof ultrasound oscillation was switched on when the transducer was takenout of powder. In this way the electrode plates and the space betweenthem were cleaned of powder residual after measurement of each parametermonitored.

At the same time it is appropriate to use longitudinal oscillationultrasound when a transducer is immersed in the test powder. As ourresearch showed, this would lead to condensing or uniform compacting ofthe test powder tested and ensure its even distribution in the spacebetween electrodes.

CLOSURE

While various embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A method of non-destructive testing for quality control of powderedmaterials having dielectric properties, said method based on the use ofelectromagnetic capacitance techniques, and comprised of the followingsteps: switching on a generator of an electric field driving atransducer that is not in contact with a samples of the powder to betested; measurement of the self-capacitance and Q-factor of theresonance circuit with a capacitive sensor, which is located in thelocal ambient environment; obtaining the computed operational parametersof said transducer electric circuit and sending them to the main unitfor processing, storage, analysis and display; switching off saidgenerator of electric field for said transducer without sample powder;positioning said sensor on the surface of the material to be tested,switching on an impulse generator to provide power for a vibratingelectromagnetic transducer and/or immersing the sensor into the materialto be tested; shutting off of ultrasound generated for the vibratingelectromagnetic transducer after the sensor has been fully immersed inthe material to be tested; switching on said generator of electric fieldfor the transducer after said transducer has been fully immersed in thematerial to be tested; measuring the phase shift caused by the effect ofthe powdered material being tested on the real and imaginary componentsof the impedance of said capacitance sensor; measuring the frequencyshift of the resonance circuit, containing said capacitive sensor,caused by the effect of the dielectric permeability of the powderedmaterial upon the capacitance of said sensor; determining capacitance ofsaid sensor, which has been fully immersed in powdered material, by thevalue of the frequency shift; determining the appropriate correctingsignal for the phase shift; determining the informative signal, whichdescribes the magnitude of dielectric losses in the field of said sensorin contact with the powdered material to be tested; transformingparameters of said sensor with powder in said electronic circuit of saidsensor and sending them to the main unit for processing, storage,analysis and display; removing the sensor from the powdered materialbeing tested; switching on the source of ultrasonic oscillation whileremoving the sensor from the powdered material to clean electrodes ofthe sensor of residual powder until the values of Q-factor of saidresonance circuit, together with said capacitive sensor, reaches apredetermined threshold values; making measuring from other areas on thesurface of powdered materials and obtaining averages for all saidmeasurements; determining the of quality of the powdered material bytaking the average of the informative signal value for all measurements.2. Method as in claim 1, wherein, the depth of immersion of saidcapacitive sensor into the powdered material is measured based on thecapacitance value of the auxiliary capacitive sensor with the coplanarelectrodes of the sensor being placed on the side surface of the workingcapacitive sensor case, wherein prior to immersion of said sensor, thesource of ultrasonic vibrations is switched on.
 3. Method as in claim 1,wherein the correcting signal is measured by the capacitance value of afilled capacitive sensor, according to a calibration curve or look-uptable of test measurement values, which have been previously made on agiven type of powered material having different moisture contents. 4.Method as in claim 1, wherein after said sensor is filled up withpowdered material, said generator of electric field with a load isswitched on according to the signal produced by an auxiliary sensor, andmeasurements are made of parameters of the transducer immersed inpowdered material wherein the source of ultrasonic vibration is switchedoff during the measurement.
 5. Method as in claim 1, wherein themeasuring transducer is taken out of powdered material and at the momentwhen the transducer lower end leaves the powder surface, the ultrasonicgenerator is switched on.
 6. Method as in claim 1, wherein theultrasound vibration source remains switched on after the measurement ofthe Q-factor of the capacitive sensor has been made and the sensor hasbeen removed from the powdered material, until the value of Q-factorcomes up to the threshold level, after which then a signal is generatedto switch off the ultrasonic vibration.
 7. Method as in claim 1, whereinthe determining threshold value of the Q-factor of the capacitive sensorfor the value of the Q-factor of the unloaded resonance circuit with asensor is carried out wherein the unloaded Q-factor is obtained prior tofirst immersion of said sensor into powdered material and adjusted usinga correction value in accordance with properties of the powder beingtested.
 8. Method as in claim 1, wherein sensor circuit signalprocessing, storage, and analysis are carried out by an appropriatesoftware program using the required algorithms to calculate the valuesof the informative parameters, and provide for their display on asuitable read-out device.
 9. Method as in claim 1, wherein the computedvalues of Q-factor, capacitances and for computed values of dielectricdissipation, relative change of Q-factor and/or dielectric permeabilitycan be used as an informative parameters, the value of which indicatethe quality of the powdered materials.
 10. Method as in claim 1, whereinthe optimal frequency, which ensures the largest possible differencebetween informative signals from the sensors with and without the powderbeing tested, is selected and the source of the electric field for theworking capacitance sensor is set to generate said optimal frequency.11-18. (canceled)